I understood that actual fasting was pretty much a poor people thing for catholics at least. i.e. That rich lords/merchants would just pay some repentance fee to their local church and could eat whatever they wanted (except for Good Friday)?
That being said, I've never heard of replacing the fasting days with a 'repentance fee'. I'm not going to say it's never happened because there have been billions of Catholics, and probably millions of priests so maybe someone did that somewhere. However, under normal Catholic doctrine it's impossible to pay to repent. God's mercy is freely given without money.
Since fasting and abstinence are a matter of Church Law and not Divine Law, the practices can be the subject of a dispensation or commutation by an authority such as the pastor. For example, on St. Patrick's Day in these United States, when it falls on a Friday (as it did this year) it's customary for bishops to grant dispensations for the faithful to enjoy corned beef, or a commutation, to pray the Rosary and then enjoy corned beef (or pork tempura).
Whether any given pastor could be influenced by the money from a "rich lord/merchant" is left as an exercise for the reader, but if practically all his parishioners are tithing and paying his salary anyway, then what can you do?
And then there absolutely were some (many, maybe) nobles and merchants who were genuinely devout. Even emperors, like Louis the Pious. I'd imagine they tried a little harder.