A Muslim had a business contract

Q: I own a spare parts store and I sell goods to individuals and companies. The individual buyers come themselves to buy the spare parts they need, while companies send some employees to buy what is needed. The employee who is responsible for buying the parts is called the "buying officer" and he comes to me to buy the spare parts that his company needs. Before buying anything, he starts negotiating (in a threatening manner) his tips, which is an additional amount of money he takes from me apart from his monthly salary from the company. If I refuse to pay, he threatens to buy from another place. Bear in mind that I offer my clients, whether they are individuals or companies, a special deduction to the official market price of the general agent in the Kingdom of Saudi Arabia, to encourage them to buy from me. After much talk with the company buying officer, I agree to pay him (Part No. 15; Page No. 345) but I take the money from the deduction I offer to his company. I used to pay the officer from the money I was going to pay to his company. To be honest, I did what is worse. I used to give this offer to other buying officers to gain their loyalty and tempt them to buy just from me. Here is an example of what used to happen: when a company wishes to buy spare parts for 100 riyals, I give them a 10% deduction but when the buying officer asks me for his tips, I stop giving the company the 10% deduction offer. I give them a 5% deduction and give the remaining 5% to the officer. Since companies demand an invoice, I write two invoices:The invoice given to the buying companyand the one that I keep.The price is 100 riyals with a deduction of 5 riyals making the due payment 95 riyals. The buying company pays 95 riyals.The price is 100 riyals with a deduction of 10 riyals making the due payment 90 riyals. I get only 90 riyals in reality.The 5 riyals' difference between the two invoices is paid by the company to its employee while it thinks it has paid it to me. This would not have happened had I not fabricated the invoice. Now Allah has blessed me and I performed Tawbah (repentance to Allah), all praise be to Allah. I stopped doing (Part No. 15; Page No. 346) anything that I used to do before and made me feel uneasy. Is performing Tawbah enough or is there something I have to do to expiate for my sins? If I have to do something, let me clarify some points related to the companies I deal with: (i) I cannot give back the money unless there is a difference and this difference only appears in the original invoices of the buying company. (ii) Since the invoices go back to 1983, 1984 and 1985, some companies get rid of them and thus I cannot prove having a difference to repay. (iii) Some of them are foreign companies; i.e., Korean, and they have already left the Kingdom. (iv) There are many companies bearing similar names and it is hard to address the concerned one. (v) I have given back the money to some companies but it was not an easy task, as the employees do not facilitate things for me. They say they cannot find such old invoices and the only way is to meet with the company owner. This alarms the employees and raises doubts about them. After much effort, I meet with the company owner and inform him of the whole story. It is embarrassing to answer questions about the concerned buying officer (Part No. 15; Page No. 347) and it becomes impossible to point at the person responsible. Finally, I hope from Allah to guide you to answer me regarding this matter. May Allah reward you good!