I wonder if your example is a different kind of trade. When we trade time in a leisure situation, we're trading something more immediate and tangible. Whereas the transactional nature of the grandparent comment is more describing something more distanced.

An example of this that was told to me is imagine you're going to dinner with the in-laws (or maybe your best friend's parents). What would their reaction be if you took out your wallet and offered to pay them cash for the meal? As opposed to offering to e.g. bring a bottle of wine or helping to set the table?

Offering to pay might be the "transactional" trade meant by the grandparent. While offering to contribute [food | labour | goodwill] is more of the trading time in a leisure situation.