Is it permissible for a sale to be conducted by paying 50% of the amount and manufacturing the materials abroad, with the remaining balance collected after one or two months? And is the salary earned from these transactions lawful?

This contract is called an "Istisna'a contract" (manufacturing contract). It is an agreement between the buyer and the seller for the sale of a manufactured item with agreed-upon specifications. It is not a condition for this contract to pay the entire price or part of it upfront; rather, it may be paid in advance, deferred, or in installments, depending on the agreement. Therefore, there is no harm in working with this company.