When Jamie and the others set off in Season One to collect the rents, he has no idea that he is about to become a political pawn and unwillingly support Dougal’s Jacobite agenda.
Even though Dougal’s speeches are in Gaelic, we (like Claire) catch on to what he’s up to. And to bolster support for his cause, he uses Jamie’s scarred back to display his hatred for the English and contribute to his political campaign.
Jamie is caught off guard when Dougal walks over to him and rips the shirt from his back to illustrate his point to the villagers gathered together “for a dram.” Jamie is in a precarious position. He’s an outlaw (accused of murder) hiding from the English, and Castle Leoch is the only haven for him. Since Colum can’t travel, Dougal is the man in charge on the road; if Jamie is to remain safe, he must tread carefully around Dougal and do his bidding. It’s hard to show restraint, but Jamie does, and even stops Murtagh as he rises to confront Dougal.

Meeting over, monies collected. Who will mend the shirt? The ripped and travel-worn garment is tossed around while Claire and Dougal argue about it. Condescending as ever, Dougal tells Claire to, “Be a good lass, get a needle and thread and mend that.” Claire, being Clare, isn’t about to be spoken down to or ordered about. She refuses, then relents. Just then, Jamie steps in, grabs the shirt, and says he’ll mend it himself.
It’s about so much more than the shirt. It’s the posturing and clashing of personalities and values. Dougal wants to run the show and order everyone about (yes, he’s War Chief of the clan, but he’s taking it to the extreme). Claire is independent, strong-willed, and not about to be ordered around by Dougal or anyone else in the 18th century.
Jamie’s back has been exposed for all to see and for Dougal to use for his political purposes. We already know Jamie is reluctant to bare his back, even to Alec, the castle’s Master of Horse. Alec knows Jamie’s been flogged, but he’s never seen the actual scars, and Jamie wants it kept that way. Claire first saw it when she tended to his gunshot wound, and Jamie is okay with that. He says she lets him know she feels sorry for it while not making him feel pitiful about it. Jamie’s back is highly personal to him, and for someone to expose it and use something so painful and degrading as being flogged (Dougal was there and witnessed it, by the way) brings the conflict between them to a new, tense level.
Jamie and Dougal exchange words one evening in camp when Dougal tries unsuccessfully to rally Jamie to his cause. “My neck is my own concern,” Jamie says. “So is my back.”

Dougal responds, “Not while you travel with me, sweet lad.” Jamie has no choice but to restrain his temper (he pounds his fists into a tree instead) because he knows, “a man has to choose what’s worth fighting for,” and this confrontation with Dougal isn’t worth the fight.
So, the shirt is mended in time for the next “event.” This time, Jamie decides how it will play out. Instead of letting Dougal rip it from his back, he removes it himself. It may seem like an insignificant change, but it’s an opportunity for Jamie to stand up to Dougal in front of everyone, assert himself, and do things his way.

The conflict between them is far from over, but the shirt’s powerful, political role appears to have fulfilled its purpose.