As a Govt. Office can make Advance payment to supplier for future supply

Government office wants to make advance payment(Future Supply) to supplier then, how can make the advnace payment ?

(1) Can Govt. office make advance payment to supplier (A) before receiving Receipt Voucher ? or (B) After Receiving Receipt Voucher ?

(2) If Govt. office wants to make advance payment before receiving Receipt voucher from Supplier then, on which basis (document)/Invoice/Deman/claim can make the advance payment to supplier ?