The send and receive cash buttons are located in the "Cash Drawer" section of the POS tab.
Transferring cash between registers is necessary on occasion and is quite simple to do. You might notice that these windows bear a large resemblance to the Opening/Mid-day/Closing windows. Using them is very similar and makes transferring cash easy. Even if you're only swapping equal amounts of cash between drawers, doing a transfer through this screen prints a receipt and gives you a paper-trail to help you keep track of transfers if you find yourself doing them often.
The major difference between the drawer count windows and the cash transfer windows is the "Sending to register #" text field at the top. This requires you to designate the register that the money is either being sent to or received from. Aside from this, the rest of the window is the same as the drawer-count windows. Simply enter the quantity of each denomination of cash being transferred and select "Send Cash Out." (For example, if you are sending $100 in $20 bills, you would type a 5 in the Twenties box.) A receipt will print out with the amount of cash transferred listed on it.
Typically, you will match up a "Send Cash" from one register by running a "Receive Cash" on the register to which the cash is being transferred, to ensure that the transfer balances correctly and all money is accounted for in both drawers.