I often find that I have to remove some of the elastic in the bottom of a jersey as well as in both arms, especially if it is a mans size that they have "adapted" or decided to label as a womans. It isn't hard if you have basic sewing skill, just a bit fiddly. If the sleeves are set in perhaps you could simply remove the sleeves altogether and have a sleeveless jersey. I have done this with several as well.
As for feeling obligated to ride since they have given you the jersey. I would not worry about it but just wear it around riding and if anyone asks you where you got it, talk about the ride and the sale and where you got it. Wordof mouth advertising, don't you know.
marni
Katy, Texas
Trek Madone 6.5- "Red"
Trek Pilot 5.2- " Bebe"
"easily outrun by a chihuahua."