The MacBook would be fine with such a charger. It will pull as much power as the charger says it is allowed to and no more and the charger (short of an outright blowing up style failure) can't push more into the MacBook than it needs.
It is entirely allowed for a USB power delivery source to change the available wattages and voltages. Many cheap chargers will do so by just resetting all of the ports and renegotiating from scratch but the specification does allow on the fly renegotiation of an existing agreement.
The other current answer seems to be focused on the common misconception that chargers push power into devices and explaining that that is not the case. My interpretation of the question here is the case of a multiport charger that limits the power available to certain ports depending on the number of devices connected. Some of them may allow 100 watts if only one USB PD device is plugged in but only 60 and 30 for each of two ports if two devices are plugged in.
https://www.eevblog.com/forum/projects/renegotiation-of-an-established-usb-pd-contract-as-source/