To what extent is it permissible to receive the remainder of the phone’s price and dispose of it, and what is the ruling on the money that was received and disposed of, given that the buyer was not informed of the seller’s waiver of the price?

It appears from the question that the questioner bought the phone as an agent for his friend. Thus, the price of the phone is a debt owed by the friend to the shop owner. If the shop owner waives this debt, the friend's liability is cleared, and it is not permissible to take any money from him. It is permissible for the questioner to inform the shop owner that the debt is on his friend; if the shop owner wishes to gift the debt to the questioner, he may do so. The payment deadline must be known in an installment sale.