raise. 500 already in there is enough to cover the 300 bet.
Here are the TDA rules, 46c2 covers this scenario:
Quote:
46: Prior Bet Chips Not Pulled In
A: To avoid confusion, players with prior-bet chips not yet pulled in who face a raise should verbalize their action before adding chips to the prior bet.
B: If facing a raise, clearly pulling back a prior bet chip binds a player to call or raise; he or she may not put the chip(s) back out and fold.
C: If new chip(s) are added silently and the bet is unclear to the house, the call and raise rules 41-45 apply as follows: 1) If prior chips don’t cover the call AND are either left alone OR fully pulled back, an overchip is a call and multiple new chips are subject to the 50% raise standard (Rule 43). 2) If prior chips are partly pulled back OR if prior chips cover the call, the combined final chip bet is a raise if reaching the 50% standard (Rules 43 & 45), if less it is a call. See Illustration Addendum.
