Terumos 7:3

If a person fed terumah to his minor children or his indentured servants, or if he ate terumah of produce from outside of Israel, of if he ate smaller than an olive-sized portion of terumah – in all of these cases, he repays the value of the terumah but he does not add a fifth. The produce he repays with is chullin (non-sanctified); if the kohein wishes to refuse it, he may.

Terumos 7:4

The general rule is that whenever one repays the value plus a fifth, what he repays is terumah and the kohein may not refuse it. Whenever one one repays the value without adding a fifth, what he repays is chullin and the kohein may refuse it.