Are the described financial transactions, which involve arranging loans with interest (riba) or fictitious sales of goods to rectify the transaction, and transferring phone credit at a discount, considered permissible? And do I incur sin by participating in them, and what should I do if there is sin?

All the transactions mentioned in the question involve usury or circumvention of it. The first transaction is an explicit usurious loan. The second is a forbidden 'inah sale (buying goods on credit and then selling them for cash at a lower price). The same applies to the third if the buyer of the balance at less than its value is also its seller. Therefore, sincere repentance is obligatory for assisting in these forbidden transactions, and sin encompasses everyone who helps in them in any way, based on the Almighty's saying: "And do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty." It is advised to cease working with the aforementioned person.