Is it permissible to demand the full agreed-upon amount (3750 riyals) or the market price (412 riyals) when a customer disputes the price with a print seller after the work has been completed?

Among the conditions for the validity of ijarah (leasing/hiring) is that the ujrah (rent/wage) must be known, because lack of knowledge regarding it constitutes jahalah (ignorance/uncertainty) and gharar (excessive risk/deception). The Prophet, peace and blessings be upon him, forbade gharar. Consequently, an agreement for a wage less than what was previously paid is an invalid ijarah due to the jahalah in the wage. In this case, only ujrat al-mithl (the customary/fair wage) is due, which is the estimation of experts for the market price of such work. The ujrat al-mithl could be 55 halalah or 3 riyals or somewhere in between, depending on the market. Your statement to your friend on the phone that the wage is three riyals does not obligate him unless he accepts it.