If somone had an agreement to pay money (e.g. repay a loan) and has violated that agreement by not making payment in full on time, you may sue him. Based on what you write, you would seem to have a case: the elements of a case are 1) proof of the agrement; 2) the terms of the agreement; 3) evidence that the agreement was vioalted. Evidence can be provided by a contract, promissory note, other documentation (such as emails, letters, text messages, etc.), bank statements, testimony, etc. It seems from what you write that all the elements should be there; the fact that you have an email from his acknowledging what he still owes you should be very helpful to you. Good luck.