Indian contract act 1872,Stifling prosecution OR VALID

You just ask B to record the settlement in court in pending case and insteadof withdrawing the case make a request the court for 3 months so B could comply with its commitment. Alternatively instead of withdrawing the case you can get the suit stay sindie so that if B does not comply with its commitment you can get the case reopened. You can also opt for consent decree.

When B has realized that he/she is guilty then what is the need of such deferred payment?

And another agreement?

Let B pay in full and on the spot.

28. Agreements in restrain of legal proceedings, void –

1[***]Every agreement, by which any party thereto is restricted absolutely from enforcing his rights under or in respect of any contract, by the usual legal proceedings in the ordinary tribunals, or which limits the time within which he may thus enforce his rights, is void to the extent.

As per  thisdefinition B has not absolutely restrained A from filing a suit forever. Since B has failed to pay 10000+5000 if A withdraws case against him.  Let us assume A has withdrawn the case with the hope thsat B would pay 15000 which is obviously more than what is due to A. If B fails to pay this amount within  3 months promised time A can again file a suit for recovery of his Rs,15000 etc. Here B has not  absolutely restrained A from proceeding legally to recover the money due to him. So B's action doesn't amount to restraint of legal proceeding or stifling  A's  prosecution. Hence B is not guilty of restraint of legal proceedings of A and therefore section 28 indian contract Act is not violated by B.