The best wealth is that which benefits you. خير مالك ما نفعك

خير مالك ما نفعك
قال أبو عبيد: العامة تذهب بهذا المثل إلى أن خير المال ما أنفقه صاحبه في حياته ولم يخلفه بعده. وكان أبو عبيدة يتأوله في المال يضيع للرجل فيكسب به عقلاً يتأدب به في حفظ ماله فيما يستقبل. كما قالوا: لم يضع من مالك ما وعظك.
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The best wealth is that which benefits you.
Abu Ubaid said: The common people understand this proverb to mean that the best wealth is that which its owner spends during his lifetime and does not leave behind. Abu Ubaidah, however, interpreted it as referring to wealth that a person loses, thereby gaining wisdom and learning to manage his wealth in the future. As they also say: "Your wealth is not truly wasted if it teaches you a lesson."
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المثل العربي «خيرُ مالِكَ ما نفعَكَ» معناه: قيمة المال ليست في كثرته، بل في مقدار ما يعود عليك منه بالنفع. وقد فُسِّر بطريقتين قديمًا: خير المال ما انتفعت به في حياتك، لا ما تركته دون فائدة.
حتى المال الذي ضاع ثم علّمك حسن التدبير لم يذهب سدى، لأن العبرة بالنفع الذي عاد عليك من التجربة.
وأقرب المقابلات في الإنجليزية هي: Wealth is of no use unless it is used. 

أفضل مقابل لا فائدة من الثروة ما لم تُستعمل.
Money is only as good as the good it can do. قيمة المال فيما يحققه من خير ونفع.
Use it or lose it. استعمله وإلا فقدت فائدته.
قريب من حيث إن الشيء لا قيمة له إن لم يُنتفع به، لكنه أعم من المال.
It's not what you have, but how you use it that matters. ليس المهم ما تملك، بل كيف تستخدمه.
يوافق المعنى وإن لم يكن مثلًا تقليديًا.
إذا أُخذ بتفسير أبي عبيدة
فأقرب تعبير هو: Experience is the best teacher. التجربة خير معلم.
لأن خسارة المال قد تكون نافعة إذا أكسبت صاحبها خبرة تمنع خسائر أكبر مستقبلًا.
أفضل مقابل
إذا أُريد المعنى الأشهر للمثل، فأقرب مقابل هو:
Wealth is of no use unless it is used.
لأنه ينقل الفكرة الأساسية: أن خير المال ليس ما يُملك، وإنما ما يُنتفع به.
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The Arabic proverb "The best of your wealth is that which benefits you" means: the value of money is not in its quantity, but in the amount of benefit it brings you. It was interpreted in two ways in the past:
The best wealth is that which you benefit from in your life, not that which you leave without use.
Even money that is lost but teaches you good management skills is not lost in vain, because the lesson is in the benefit you gain from the experience.
The closest equivalents in English are:
Wealth is of no use unless it is used.  

Best equivalent: Wealth is useless unless it is used.
Money is only as good as the good it can do.
The value of money lies in the good and benefit it brings.
Use it or lose it.
Use it, or you will lose its benefit.
Similar in that something has no value if it is not used, but it is broader than just money.
It's not what you have, but how you use it that matters.
It's not what you own that matters, but how you use it.
This agrees with the meaning, even though it is not a traditional proverb. If we take Abu Ubaidah's interpretation,
then the closest expression is:
Experience is the best teacher.
Because losing money can be beneficial if it gives one experience that prevents greater losses in the future.
Best equivalent
If we want the more common meaning of the proverb, the closest equivalent is:
Wealth is of no use unless it is used.
Because it conveys the basic idea: that the best wealth is not what is owned, but what is used.