Usually, we express future action by adding the word 'akan' before the verb. Your sentence will be translated to indonesian as 'kalau dia datang saya akan pergi'.
This construction with “if” and “will” isn’t future tense; it’s conditional. The word “will” is needed here for the English to be correct. For an example, see https://www.ef.edu/english-resources/english-grammar/conditional/