Is it permissible for a Muslim to take personal leave from the Communist police to travel safely and be treated economically within Communist China? And what is the Islamic evidence for that?

The basic principle is that it is permissible to deal with disbelievers who are not hostile towards Muslims, within the bounds of what is permissible in Islamic law, such as buying, selling, or lending. However, if this dealing leads to harm, such as temptation in religion or loss of wealth, then it is not permissible to deal with them, based on the Prophet, peace and blessings be upon him, who said: "There should be no harm nor reciprocating harm."