Dear SR J,
This is system standard behavior that it always make available down payment amount to be used in for another commitment. You can always change it by making two changes in the system, after which the system won't be reducing the commitment of down payment at all.
1. Exclude Value Type Down Payment (61) and Down Payment Request (58) from AVC check; and
2. Implement Note 1126105 in system.
I hope this will serve the purpose.
Regards,
Amar Lal