Is placing milk in the mosque considered selling within it, even though the agreement, contract, and handover of money occur outside the mosque?

The scenario mentioned in the question does not fall under the meaning of the prohibition of selling in the mosque. It is devoid of the cause for prohibition. The intended meaning behind preventing buying and selling in the mosque is to prevent it from becoming a marketplace, losing its sanctity, and disturbing those praying and those in i'tikaf.