What is the ruling on setting a 25% rate on the remaining amount after paying more than half the price of a device that someone bought for me?

The apparent meaning of the question is that the 25% is an interest charged on the debt in exchange for deferment, and this is precisely usury (riba). If the questioner wishes to make the transaction permissible, then the first party should purchase the device for himself and then sell it to the second party for whatever price he desires, whether in cash or on credit, or partly in cash and partly on credit, with the profit he desires. This transaction is called Murabaha.