What is the ruling on earning and financial dealings with a supply company that has cheated in product quality and failed to adhere to agreed-upon specifications (especially galvanization testing and product dimensions), and what must the partners do to atone for past profits and for the future of the capital and its related zakat?

You have done well in repenting from cheating and deception in selling. Islamic law forbids this and encourages honesty, as stated in the Hadith (definition): "If they are truthful and clarify, they will be blessed in their transaction. And if they lie and conceal, the blessing of their transaction will be erased," and his (peace be upon him) saying: "Whoever cheats is not of me."

As for the money earned from sales that involved cheating, you must return the excess amount resulting from the deception to the buyer. If it is impossible to identify him or reach him, then give that amount in charity on his behalf. If you do not know the exact amount of illicit money, then strive to estimate it to the best of your ability until you are confident that you have given out the due amount or more, as mentioned by Imam Al-Qurtubi. You must adhere to honesty in all your dealings to attain Allah's blessing in your livelihood.