If tenants cannot make partial payments, cannot edit the payment amount, or can only pay the exact rent amount, the Allow Partial Payment option is turned off in the tenant portal settings.
How to Fix It
Check and enable partial payments at all three levels:
Company level — Go to Settings > Tenant Portal > Permissions and enable Allow Partial Payment
Property level — Open the property's Settings > Tenant Portal > Permissions and enable Allow Partial Payment
Lease level — Open the lease's Settings > Tenant Portal > Permissions and enable Allow Partial Payment
All three levels must allow partial payments for tenants to be able to pay custom amounts.