There absolutely is an example in the Bible, where God actually does command a situation of polygamy ---in the New Testament, even.

1_Corinthians 7:10-11 & 27-28.

In 1 Corinthians 7, the Apostle Paul differentiates when he is making his own "recommendation" (in verses 6, 12, and 25) and when he is expressing the "commandment of the Lord" (verses 10-11). Indeed, in verses 10-11, Paul clarifies that the instruction in those two verses is the "commandment of the Lord". (It should therefore also be noted that the other areas in which he clarifies as being only his "recommendation" can NOT be used to otherwise and incorrectly assert that God Himself is creating some sin or doctrine. After all, Paul's ultimate "recommendation" therein is celibacy!)

With that realized, it is clear for readers of the Bible that Paul makes it emphatically clear that verses 10-11 are different. Namely, verses 10-11, in the exact way in which thay are actually written, are the "commandment of God".

"And unto the married I command, yet not I, but the Lord, Let not the wife depart from her husband: But and if she depart, let her remain unmarried, or be reconciled to her husband: and let not the husband put away his wife." 1 Corinthians 7:10-11.

Paul further specifies that that above "commandment of the Lord" was only addressed to believers-married-to-believers. In the next verses (i.e, 12-16), he clarifies that he is subsequently addressing believers-married-to-unbelievers, and that that subsequent instuction is not the Lord's words, but his own again.

Verses 10-11 show that, if a believer WIFE leaves her believer HUSBAND, the

• believer WIFE is commanded of God to either:
  remain unmarried, or
  be reconciled back to her husband
  
  
  
• believer HUSBAND is commanded of God to:
  not put away any wife, and to
  let any departed wife return back to him

The key point is that the HUSBAND is NOT given the same commandments of instruction. Only the WIFE is commanded to remain unmarried, but the HUSBAND is not given that commandment. He is commanded of God to let her be married to him, either way!

Accordingly, the HUSBAND is of course, still free to marry another wife. That fact is further proved by the later verses of 27-28d.

"Art thou bound unto a wife?
  seek not to be loosed.
Art thou loosed from a wife?
  seek not a wife.
But and if thou marry, thou hast not sinned;
and if a virgin marry, she hath not sinned."
1 Corinthians 7:27-28d.

The Greek text of verse 27 is clearly only addressing married men --whether or not the wife has departed.

As such, the married man whose wife is still with him does not sin when he marries another wife (who is not another's wife). And likewise, the married man, whose wife has departed from him, he also does not sin when he marries another wife (who is not another's wife).

And herein comes the "commandment of the Lord", of polygamy, as in the following situation.

A believer WIFE departs from her believer HUSBAND. She is commanded of God to remain unmarried, per verses 10-11. Her HUSBAND, however, then subsequently marries another wife (who is not another man's wife). The HUSBAND and the new wife have not sinned, per verses 27-28. The departed WIFE then seeks to be reconciled back to her HUSBAND.

In that situation, verses 10-11 show the following instruction as the "commandment of the Lord". The HUSBAND is commanded of God to let the departed wife be reconciled back to him. AND.... he is commanded of God to not put away a wife, including the new wife.

As such, verses 10-11 show that it is an outright "commandment of the Lord" of polygamy for the family in that situation.

1 Corinthians 7:10-11 is indeed a Commandment of God --- in the New Testament --- that, when a previously-departed believer wife returns, her believer husband and his new (believer) wife (from verse 27c-28d) MUST let the previous wife be reconciled to her husband.

There truly IS a "commandment of the Lord" for a situation of polygamy to be found in the Bible ---and it's in the New Testament Scriptures, as well!