When transaction moves from commitment to obligation, if there difference it can't return to budget?

When a transaction transitions from commitment to obligation, is it possible for the difference between the two amounts to be returned to the budget pool only with approval?

Example:
A PR is created for 100,000. When the PO is issued for 80,000, the initial 100,000 commitment is closed, and an obligation for 80,000 is opened. The customer has requested that the remaining 20,000 not be returned to the budget pool automatically, but only with explicit approval.

Any idea or workaround is appreciated