A rigid master was the law

Back in October 2006, I posted a bit of poetry on Law and Grace, possibly written by John Bunyan, Ralph Erskine or John Fischer. Last month I happened upon a post -- Run, John, Run -- The author wrote, "I’m not sure, however, Bunyan ever wrote this profound and pithy summary."

I found similar poems to the one I posted that October, but have not found an original source showing this is by Bunyan. Ralph Erskine has four lines that are similar, plus two lines in another poem with the same sentiment:

A rigid master was the law,
Demanding brick, denying straw;
But when with gospel-tongue it sings,
It bids me fly, and gives me wings.

Gospel Sonnets; Or, Spiritual Songs, In Six Parts, (The Eighteenth Edition, Ralph Erksine, Edinburgh: W. Darling, 1782, p. 305.)

By promised aid, but lo; the grievous law
demanding brick, won't aid her with straw

(The Sermons, and Other Practical Works, Volume X, Ralph Erskine, London: R. Baynes, 1821, p. 26; the "rigid master" poem is on p. 221)

I'm not sure when Gospel Sonnets; Or, Spiritual Songs was first published, but Ralph Erskine died in 1752, so what he wrote must have been written before then! "Faith, without trouble or fighting, is a suspicious faith; for true faith is a fighting, wrestling faith."

"Run, John, and work" appears as part of a 5 stanza hymn by John Berridge in his Sion's Songs, or Hymns: Composed for the Use of them that love and follow the Lord Jesus Christ in Sincerity (John Berridge, London: Vallance and Conder, 1785, Hymn 150, Page 203). When Charles H. Spurgeon mentioned this hymn, he credited it to Berridge (as did William Gadsby in his hymn book). Both Spurgeon and Gadsby were quite familiar with Bunyan's works. It would seem they wouldn't have mentioned that if it were by him.

Hymn 150
1. The law demands a weighty debt,
And not a single mite will bate;

But gospel sings of Jesu’s blood,
And says it made the payment good.

2. The law provokes men oft to ill,
And churlish hearts makes harder still;
But gospel acts a kinder part,
And melts a most obdurate heart.

3. Run, John, and work, the law commands,
Yet finds me neither feet nor hands;
But sweeter news the gospel brings;
It bids me fly, and lends me wings.

4. Such needful wings, O Lord, impart,
To brace my feet, and brace my heart;
Good wings of faith, and wings of love
Will make a cripple sprightly move.

5. With these a lumpish soul may fly,
And soar aloft, and reach the sky;
Nor faint nor falter in the race,
But cheerly work, and sing of grace.

It is possible that both Erskine and Berridge based their pieces on an earlier work, or perhaps this portion of Berridge's hymn is derived from Erskine.