How should I deal with a colleague talking on my behalf? Our Chair is writing a major proposal and me and my colleague are on-board (both non-tenured)... Chair has asked for a meeting next week to work on the bid for a centre and this colleague of mine has replied back to both of us accepting the meeting and on our behalf and has suggested working two-three full days for the Chair. 

I replied to the Chair saying "sure lets meet" and didn't acknowledge anything else that was said. 
I am tempted to write to the my colleague and say I know they had the right intention but please never ever talk on my behalf again as its not professional. Is this the right thing to do? 

I am worried if I don't react this will continue as I am new in my current job. On the other hand I am worried if I react by sending an email or verbal communication and asking them not do it they might just forward/talk with the Chair with some lame excuse like suggesting that I am really busy and unhappy and they will do it themselves to make me look bad or with the genuine intention of correcting their mistake which in both cases will make me look poor! 

What should I do?
I'd suggest putting in those days - they may or may not be lost, but losses occur*. No use crying over spilt milk. Writing the time down as lost means, you can be happy if something good comes out of them at the end. OTOH, aking a fuss creates a serious risk of a trench war that would cost much more time.    

However, I'd also talk to him in a calm minute. I may be taking a line saying that he luckily foresaw the way my opinion went, yet I'd wish him to quickly ask me beforehand in the future.
Talkin calmly may need that you know exactly why you are so angry that you have to post in a public Q&amp;A site. 

* partiuclarly in writing proposals. There's a rumour here in Germany that the Bundesrechnungshof once estimated that for different kinds of public grants from a national economic point of view (i.e. including all proposals that are written: accepted as well as rejected) about as much money/effort goes into writing grants as is distributed by the different grant programs (estimates ranging from 1 : 2 to 2 : 1 are mentioned).