There were young kids walking round every town in the UK with juventus shirts with Ronaldo on the back.Commercial contracts are already agreed so that's a no -starter unless he stays long term and those contracts are renegotiated. Which is unlikely given his age.
Go through any historical data you like, shorts sales don't significantly increase due to players joining, the only thing that changes is what name goes on the back. Additionally, city get a tiny percentage (7.5-12%) of the profit on any shirts sold so don't make any significant dough from it.
The rags are the top sellers with 3.25m per year, Madrid are close by and saw no downturn in sales when Ronaldo left. Juve sold a total of 500k Ronaldo jerseys the season he joined, but these weren't additional sales, just sales with his name on the back. It's the biggest lie in football
They weren't buying juventus shirts before he signed.