Is there a hotkey sequence to send minerals or gas to your ally? Alternatively is there a way to bind hotkeys to the action?
Press Ctrl + F12 to summon the Team Resources menu. Should you want to send many resources at once, use the scrollwheel to increase the resource count (it's far faster than clicking the GUI button). Holding the Ctrl key while clicking the increase or decrease buttons apparently uses increments of 100.
I don't believe there is any in-game way to bind a pre-determined amount to a key.
You can rebind the resource trading menu. Found under Hotkeys > Global > Menu but like sequentiallee mentioned, I don't believe there is any in-game way to bind a pre-determined amount to a key.