Praise be to Allah.
Although we appreciate your daughter telling her family about this man’s offer and what he has promised her, we do not approve of her corresponding with and talking to a man who is not her mahram (close relative who one may not marry). Although she may have been wise and mature – praise be to Allah – there are many other girls who have fallen victim to such correspondence.
After thinking about what you have told us about your daughter and about that German man, we strongly advise you not to accept him as a husband for your daughter. The points you have listed are sufficient for a ruling of this type. The shar‘i prescription that the guardian should play a fundamental role in the marriage contract is supremely wise, lest the girl follow her whims and desires and accept any sweet talk that may make her build up false hopes. The proof of that is that you did not want to base your judgement solely on what you felt about this man; rather you also hastened to submit a question to this website. This indicates – in sha Allah – that you are a family that is fit to be in this position of responsibility, as you are trying sincerely to protect your daughter.
The Prophet (blessings and peace of Allah be upon him) advised guardians to choose those who are good and righteous to marry the females under their guardianship, namely those who are pleasing in terms of religious commitment and character. It was narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “If there comes to you one with whose attitude and religious commitment you are pleased, then marry (your daughter or female relative under your care) to him, for if you do not do that there will be fitnah (tribulations)in the land and widespread corruption.”
Narrated by at-Tirmidhi, Ibn Maajah, and classed as hasan (sound)by al-Albani in Saheeh at-Tirmidhi.
A man said to al-Hasan: I have a daughter; to whom do you think I should give her in marriage? He said: Give her in marriage to one who fears Allah, may He be exalted, for if he loves her he will honour her, and if he resents her he will not mistreat her.
How can you find out about this man who wants to marry your daughter? Here we should point out that he may really be a Muslim, and he may be sincere in his wishes, but you do not know any of that, or anything else, about him. If we assume that it has been confirmed to you that he is a sincere Muslim, the other things that you mention after his being Muslim are also sufficient to rule that he is not suitable to marry your daughter. And among these things, it is sufficient that she would be living in a non-Muslim country, and she would be studying and working in their environment and in their country, which gives rise to fear for her religious commitment and morals. Usually the differences of environments, languages and natures has a negative impact, which leads to failure of the marriage in many cases.
Hence we agree with you completely that you should reject this marriage, and we advise your daughter to forget about accepting this man as a husband. She should understand that the opinion of her family and those who have more knowledge than she does about men and environments should, beyond any shadow of a doubt, take precedence over her opinion. She should ask her Lord to choose for her the best of righteous men to be her husband, so together they can establish a family based on obedience to Allah and raise – in sha Allah – righteous children.
We ask Allah, may He be exalted, to guide her to the best of words, deeds and attitudes, and to bless her with a righteous husband and righteous offspring.
And Allah knows best.