Speaking to kicker, Plenge said: "Dani is highly regarded and is our player - and that will still be the case when we start training again after the trip to the USA and nothing happens until then.
"He played a very strong European Championship, so it was clear that other top clubs were knocking on the door. Transfers are part of the business and are a source of income. But they are also just one of many. One thing is clear: we will not sell any player for less than he is worth.
"We are not under pressure to sell. We plan commercially and also with transfer income, but if we don't generate any, for example, because an offer does not meet our expectations, we will have to generate other income."