Is it permissible for a father to buy tomatoes from farmers for his shop and then send them to his customer in Damascus, knowing that he buys such goods from the market, and is this considered a contract within a contract?

If the questioner's intent is that this transaction falls under the Hadith of the Prophet, peace and blessings be upon him: "Do not meet the incoming traders (Jalab). And whoever meets them and buys from them, then when his master (the owner of the goods) comes to the market, he has the option [to revoke the sale]," narrated by Muslim, then this understanding is incorrect. The sale mentioned in the question is a valid sale, whether the merchant buys for himself or for his client in another region.

As for what he referred to regarding two contracts in one contract or two sales in one sale, it has nothing to do with the issue. "Whoever makes two sales in one sale, he gets the lesser of them or riba (usury)," narrated by Abu Dawud. This has two interpretations: The first: is to say: "I sell it to you for two thousand on credit, and for one thousand in cash, whichever you choose to take it for." This is a void sale. The second: is to say: "I sell you my slave on condition that you sell me your horse."