It is always forbidden to test the Almighty…with one exception.  The Bible promises that if one gives a full tithe he will become rich.  And it is permissable to test that promise.  Wealth, of course, need not be in the form of material posessions.  As Pirkey Avot says, the truly wealthy person is the one satisfied with his or her lot.  There is some controversy as to whether the promise of wealth applies just to agricultural tithes or to monetary tithes as well.  The modern rabbinic giant, the Chofetz Chaim, argues that monetary tithes fall under the promise, but see the final excerpt below.

Robbi Yochanan met Reish Lakish’s young son– Tell me what verse you learned today

[The boy:] — “You shall surely tithe…” (Deuteronomy 14:22).
[“Aser t’aser” the word aser (tithe) is repeated for emphasis]

[The boy continues:] Why aser t’aser (why the repetition)?

— Tithe so that you become rich [A play on the word t’aser which is similar to Tit’asher = become rich.]

[The boy:]– How do you know?

— Go test it

[The boy:]– Is it right to test the Holy One Blessed Be He? It is written “Don’t test Hashem…” (Deuteronomy 6:16).

— This is what Robbi Hoshiah said: “Apart from this [tithing], as it is said, (Malachi 3:10): ‘Bring the whole tithe to my treasury so that there is food in my house. And please test me in this says the Lord of Hosts if I will not open the windows of Heaven and pour out a blessing on you until there is more than enough.'”
 (Ta’anit 9a)

 
The Chofetz Chaim repeats the essence of the idea in a context that obviously refers to monetary tithes as well as agricultural tithes:
 

Tithing is a matter of great importance as it is said in Ta’anis on the verse “You shall surely tithe…” Tithe so that you become rich.
In all matters, it is forbidden to test the Holy One Blessed is He. But here [with regard to agricultural and monetary tithing] it is permitted
for the reason mentioned in the text: “Bring the whole tithe to my treasury so that there is food in my house. And please test me in this says the
Lord of Hosts if I will not open the windows of Heaven and pour out a blessing
on you until there is more than enough.” (Malachi 3:10)And so it is ruled in [the Shulchan Aruch] Yoreh Deah (247) that with regard to tithing it is permitted to test HaShem, may He be blessed.(The Chofetz Chaim, in Ahavas Chesed Chapter 18).

 
In a footnote, the Chofetz Chaim notes that some authorities disagree with the ruling of the Shulchan Aruch.  Their position is that one may test HaShem with agricultural tithes only. They argue that agricultural tithing is a Biblical command and monetary tithing is not. The Chofetz Chaim, however, notes that in the time of Malachi, the source of the proof text, circumstances were such that agricultural tithing was no longer a Biblical law, but had become a Rabbinic ordinance.  He goes on to argue that tithing alone was not the core of the test for which a reward is guaranteed.
 

The reason Scripture permits testing is not in order to fulfill the mitzvah of tithing but to strengthen the hand of the Priests and Levites. There should be food in the house of HaShem in order to strengthen HaShem’s Torah. And therefore it said, “so that there is food in my house. And please test me in this says the Lord of Hosts if I will not open for you the windows of Heaven and pour out a blessing for you…” The blessing comes in the merit of giving food. It says in the Gemara in Chulin that one does not give gifts to a Priest who does not learn Torah as it says “Give the portion of the Priests and Levites in order to strengthen HaShem’s Torah.” If that is so, it’s also true for monetary tithes in our day. Their essential and optimal feature is to sustain those who labor in Torah. (The Chofetz Chaim, in Ahavas Chesed Chapter 18).

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