Gallus bet his masculinity for two weeks (gender swapping potion) if smolder dared to just enter the barn, and then jokingly he added that he would even take the same treatment if she took it all the way
 
He honestly didn’t knew the rules before hand, and even assured to his friends that she could leave in any moment if she wanted
 
He tried to run after his friends forced him take the potion, accurately she tried to run when her friends demanded her to pay his debt
 
it took some helping from applejack to bring him to pay his/her debt
 
now normally big macintosh it’s against any kind of forced arrangement, even it just a little
 
his rules are clear for everyone, because of that very reason, once you accept the treatment and enter the barn you will fulfill it. mostly because all the potions and enchantments that he needs to take for the appointments
 
but when he came back from his high and understood that the little dragones destroyed on his table didn’t knew about the rules… He was really mad
 
when her sister told him that he would need to take care of the perpetrator, well…  
this it’s just the round one
 
It doesn’t matter if he/she is resolved to pay the debt, big Mac is going to make sure that this lesson is learned

imported from https://derpibooru.org/images/2307863