Condition On Contract
The principle in business is that it should not entail anything unethical. There should be no intention or effort to deceive or harm someone. After the contract both the parties have to agree on anything which is not mentioned in the contract. In your case it is asking a favour and therefore no objections can be raised.
Riba is only in the matter of loan. Asking for a favour even in loan is kir riba. It is the compulsion to pay more than the actual amount makes it riba.