Petitioner, as the recruiter and agent of Abdul Aziz, is thus solidarily liable with the latter for the unpaid wages of respondent. This Court, through Justice Irene Cortes, in Royal Crown Internationale v. NLRC29 explains the basis thereof: …Petitioner conveniently overlooks the fact that it had voluntarily assumed solidary liability under the various contractual undertakings it submitted to the Bureau of Employment Services. In applying for its license to operate a private employment agency for overseas recruitment and placement, petitioner was required to submit, among others, a document or verified undertaking whereby it assumed all responsibilities for the proper use of its license and the implementation of the contracts of employment with the workers it recruited and deployed for overseas employment [Section 2(e), Rule V, Book I, Rules to Implement the Labor Code (1976)]. It was also required to file with the Bureau a formal appointment or agency contract executed by the foreign-based employer in its favor to recruit and hire personnel for the former, which contained a provision empowering it to sue and be sued jointly and solidarily with the foreign principal for any of the violations of the recruitment agreement and the contracts of employment [Section 10 (a) (2), Rule V, Book I of the Rules to Implement the Labor Code (1976)]. Petitioner was required as well to post such cash and surety bonds as determined by the Secretary of Labor to guarantee compliance with prescribed recruitment procedures, rules and regulations, and terms and conditions of employment as appropriate [Section 1 of Pres. Dec. 1412 (1978) amending Article 31 of the Labor Code]. These contractual undertakings constitute the legal basis for holding petitioner, and other private employment or recruitment agencies, liable jointly and severally with its principal, the foreign-based employer, for all claims filed by recruited workers which may arise in connection with the implementation of the service agreements or employment contracts [See Ambraque International Placement and Services v. NLRC, G.R. No. 77970, January 28, 1988, 157 SCRA 431; Catan v. NLRC, G.R. No. 77279, April 15, 1988, 160 SCRA 691; Alga Moher International Placement Services v. Atienza, G.R. No. 74610, September 30, 1988]30 (Emphasis and underscoring supplied; italics in the original) Petitioner argues, however, that the foregoing rule has no application in the case at bar because it applies only to one which raises the issue of nonpayment but not one which raises issues of underpayment,31 hence, the burden was on respondent to show that he was indeed underpaid. 32 Petitioner does not persuade. On repeated occasions, this Court ruled that the debtor has the burden of showing with legal certainty that the obligation has been discharged by payment.33 To discharge means to extinguish an obligation,34 and in contract law discharge occurs either when the parties have performed their obligations in the contract, or when an event the conduct of the parties, or the operation of law releases the parties from performing.35 Thus, a party who alleges that an obligation has been extinguished must prove facts or acts giving rise to the extinction. The fact of underpayment does not shift the burden of evidence to the plaintiff-herein respondent because partial payment does not extinguish the obligation.36 Only when the debtor introduces evidence that the obligation has been extinguished does the burden of evidence shift to the creditor who is then under a duty of producing evidence to show why payment does not extinguish the obligation. The lack of merit of petitioner’s petition notwithstanding, this Court finds that the appellate court’s affirmance of the award to respondent of salaries for a 13-month period, as reflected in the computation of salary differential in the decision of the labor arbiter, calls for modification. Respondent himself alleged in his Complaint-Affidavit having been underpaid for 12 months37 albeit, oddly enough, in the above-quoted prayer of his said ComplaintAffidavit, he prayed for salary differential in the amount of "SR500 per month for seven [7] months… starting the 5th month of his work or July 1992 up to February 1993 or [a total] amount of SR3,500." Respondent being entitled to a monthly salary of US$370.00, 38 its equivalent of 1,200 Saudi riyals of which has not been disputed, and his allegation that he received a monthly salary of 900 Saudi riyals for the first 4 months and 700 Saudi riyals for the 5th month until the end of the 12th month not having been successfully refuted, he is entitled to SR5,200,39 not SR5,500, representing the total deficient payment of his salaries for a 12-month period. WHEREFORE, the Decision of the Court of Appeals in C.A. G.R. SP. No. 52920 is AFFIRMED with the MODIFICATION that respondent, Willie Batomalaque, is only entitled to 5,200 Saudi riyals, instead of 5,500 Saudi riyals. Costs against petitioner. SO ORDERED. CONCHITA CARPIO MORALES Associate Justice WE CONCUR: ARTEMIO V. PANGANIBAN Associate Justice Chairman ANGELINA SANDOVAL-GUTIERREZ Associate Justice RENATO C. CORONA Associate Justice CANCIO C. GARCIA Associate Justice ATTESTATION