To allow customers to pay any amount they would like, you'll need to edit your registration program and set these options.
1. Go to Admin > Members > Registration System > Edit.
2. On the Payment Tab, select the Scheduled Payment option.
3. Do not enter or add any payment schedules, but check the Allow Partial Payments option.
Now when the member logs in to pay, they will be able to enter any amount they would like to pay against their balance.