Hi @lauracastagna 

There is no out of the box option available to enable approval for write off process. 

You may post a request on our ideas page here on community portal. 

One can use manual process to record the write off amount by,

• Create manual credit memo to write off the amount of specific invoice (non stock item linked to write off expense account)
• There wont be any link between invoice and credit memo, one may manually enter the invoice reference on credit memo).
• Apply the credit memo while receiving payment from relevant invoice. 

Hope this helps,

Regards