Continuity mistake: In the wide angle shot of Chief Gillespie approaching Virgil Tibbs at the train station, there is a dog on the platform, in between Gillepsie and Tibbs. In the next shot, a closer view of Gillespie approaching Tibbs, the dog is gone.
Continuity mistake: Gillespie does a fairly lengthy audible rewind of the killer's taped confession, but only his last two sentences ("That's all. I didn't mean to kill him") are heard when the tape is immediately played back.
Continuity mistake: Lloyd Purdey is shot by Ralph and is declared dead by one of his friend. In the last shot before the scene changes, you can see the body is breathing.
Answer: He didn't. After Ralph killed Mr. Colbert, he stole his wallet. He took the money and threw it away. Harvey found the empty wallet, and that's why the police charged him with murder.
That isn't correct. It is stated several times that Colbert withdrew $900 from the bank. Harvey said he found the wallet with $300 in it ("more money than I ever saw"). When Gillespie discovers that Sam has deposited $600 in his bank account, he assumes that this is the $600 that was taken from Colbert's wallet. He even states that Sam left the $300 in the wallet in order to trap "someone like Harvey." It is the weakest part of the plot. Why would Ralph not have taken all the money?
Deramore