The Glasgow Warriors openside flanker has played only six games after recovering from a knee injury that kept him out of action for 14 months.
"It's great to be involved again so quickly. The memory of playing for Scotland - and the dream of restarting my international career - kept me motivated during my time out.
"My last appearance before the injury was that dreadful result against the Springboks and there is something really special about having the opportunity to make my Test return against them.
"Scotland have obviously improved since then and we are confident we can compete much more strongly with them."
"I felt last week for the first time that I was getting back to somewhere close to my normal game," Donnie Macfadyen said in The Times. "Throughout the time I was battling back from injury, the thought of pulling on a Scotland shirt again was a huge motivation and I am delighted that I have got this far this quickly."
Donnie stressed he was not treating the South Africa trip as just another phase of his rehabilitation programme.
He said: "I am always wanting to be in the starting line-up, no matter who I am representing.
"I'm not suggesting I'll walk back into the team because Ally Hogg has been performing superbly on the open-side but I will be giving it my all.
"The match against the Barbarians will be the ideal way to break my way back into the scene. Last Friday, when Glasgow Warriors beat Llanelli at Firhill, was my first 80-minute stint and I felt like my old self.
"That's probably because I've been managed very carefully by the medics and physios after hitting so many brick walls during my lay-off.
"Since I came back, I've been made Warriors captain, which is a tremendous honour. I believe I am also more mature as a player and can add an extra dimension to Scotland."