@ELS Hotel rates change like airline rates. They go up and down depending on demand.
Normally large groups, like weddings, guarantee a certain number of rooms at a certain rate. This rate is normally at, or below, their average room rate so the guests are getting a decent deal. As the date gets closer, the last minute rate may be more or less than the group rate the wedding got depending on how full the hotel is.
Was your booking prepay and non-refundable? Most of the time they are because wedding has to make up any short falls (ie pay for) on the room block that they didn't release (the room block contract lays out when they have to release rooms by).
Short answer... Manager was probably keeping the couple from having to pay for your room if you cancelled and rebooked
+++++++++++++++++++++++++Now that you have some answers to your question....I wouldn't impose the stress and complications on the couple to save $75. You're probably locked in anyway, as the hotel stated. However if a professional wedding planner is handling the logistics, you might contact them and leave the bride/groom out of the discussion. IMHO.
Your original post sounds like the wedding may be in the past...I hope it all went great!