How can the financial agreement between two partners be rectified in accordance with Islamic law after fifteen years of work?

Your question encompassed several matters. First: The ruling on agreeing to invest a thousand dollars with him, on condition that the profit be 150 dollars monthly. This is unlawful, and the contract is invalid, because the transaction described is a Mudarabah (profit-sharing) partnership, and it is not permissible therein to guarantee the capital nor to fix the profit as a lump sum. Rather, it must be a known percentage of the profit that changes with changes in profit and loss.

The second matter: How the profit in this invalid transaction should be handled. The answer is: The entire profit from that amount belongs to its owner (the investor), and the worker (your friend) is entitled to a fair wage (اجرة المثل), because it is a contract for which a specified consideration is due in its valid form, thus a fair wage is due in its invalid form.

The third matter: How to correct that transaction to make it Sharia-compliant. This can be achieved by making it either a Mudarabah with the aforementioned conditions, or a partnership where you are his partner to the extent of your capital, and the profit is distributed according to the capital or what is mutually agreed upon.