The difference is that paying in installments is a purchase made for a specific agreed-upon price, albeit higher, over a set period; whereas buying on credit is a purchase of something for a fixed price, made through an interest-based loan, and as long as the loan is not fully repaid the interest will apply above the agreed-upon price. Therefore, paying in installments does not involve Riba as defined by the Sharia and is permissible, and paying on credit does involve Riba and is prohibited.
Based on compulsory precaution, one should stone the area of the original Jamrah itself – after trying to find out to a reasonable extent where it is – or as close to the original Jamrah as possible.
Masturbation is a prohibited act and should be avoided with seeking forgiveness from the Almighty Allah and repenting to Him. The prayers that the person knows that he did after masturbating and before performing Ghusl should be repeated. It is not required to make up the fasts.