What is the ruling on a merchant paying deferred bills of exchange to a friend in exchange for a percentage as a gift, in order to obtain cash liquidity?

What you have asked about has two scenarios:

The first: That it is a loan which you repay as it is, and you give the lender an additional amount upon repayment, as a gesture of generosity and without a prior agreement. This is permissible.

The second: That it is a loan with an agreed-upon repayment involving a percentage increase. This is forbidden usury (riba), and naming the increase a "gift" does not change the ruling of its prohibition.