Allow buyers to pay

[kyrers]

Allow buyers to fully or partially pay escrow transactions with the following logic:

• If no dispute;
• In buffer period:
  • But during this period, can only pay the full amount.
• For legacy transactions without timeout:
  • Always, because execute transacton will always revert. But after the buffer period starts, can only pay the full amount.
• For new frontend transactions:
  • Only if transaction has not expired.