Approval level by purchasing

Greetings,

Please, I need your opinions,

For example, the purchase order has 2 levels of approval. The first level is from $ 1 to $ 500. The second level ranges from $ 501 to $ 1,000. If you enter a purchase order for $ 600 goes straight to level 2 approval. When this happens, the field observations is locked.

If instead, the purchase order through the level 1 and level 2 then by the remarks field if active.

How I can unlock the field observations, when the purchase order goes straight to level 2 approval?

Thanks in advance!!...

Betty!