“Just keep track of how much you're spending and I'll pay you back.”

You're working on a school project and need to buy some supplies for it. Your partner on the project offers to buy the supplies. You want to pay half of the cost of supplies, but you don't want to discuss the cost of the supplies now, so you say:

Just keep track of how much you're spending and I'll pay you back.

just (do something)

In this sentence, "Just ___" has a different meaning than when you're giving a command like "Can you just stop it please?" In the situation above, "Just keep track of how much you're spending" is similar to saying "Simply keep track..." The speaker uses the word "just" to make the suggestion sound easy to do.

keep track of (something)

When you keep track of something, you remember it, record it, or write it down so that you don't forget. Things that people often "keep track of" include:

• keep track of time
• keep track of your money
• keep track of your progress
• keep track of your kids
• keep track of important events

how (much/many)(someone)(does)

This is a phrase to use when you want to discuss a number as a noun:

I don't remember how many I ate.

There's some debate over how much we should spend on advertising.

pay (someone) back

To "pay someone back" means to return money that you borrowed from them. For example, when you have to borrow money from your roommate you can say:

I promise I'll pay you back by the end of the week.

Or, in situations like the one at the top of this post, it means to give someone money when they paid for an item that you both shared, like a lunch.