Will an Army Reserve 25U who is deployed with a finance unit get sent into the field with combat units?

It's very well possible, needs of the Army can place you well outside your comfort zone.

You could get pulled to support a combat unit, and despite your MOS whatever it is, it is to provide support to the warfighter. Additionally, whatever your MOS, you are expected to be able to operate a weapon system. Everything is in the realm of the possible while deployed, I know Paralegals and Cooks who have seen combat.

Don't ever confuse yourself with an Infantryman, their job is to practice combat. You are however, a Soldier, and all Soldiers are expected to support the fight and to be riflemen if the time comes.

RTO's are valuable, and despite the 25 Useless and RC jokes you'll hear so is your skill set.

Yeah anyone could go no matter your MOS, that's why we have rifle quals and things of that nature. I'm not saying you'll be kicking down doors but IEDs and Indirect are pretty equal opportunity.

I can say that, as a 25U myself I was deployed in 09 to kohst provenance Afghanistan and there was a need for my mos with some of the combat multipliers in the AO so I was tasked out from my original unit which was an aviation unit to do some dismounted missions as an RTO. So it is possible